It is generally necessary to align sheet-shaped materials which have been delivered, to a precise position for a subsequent processing step, such as printing, cutting, punching, binding or the like, since the exact position of the sheet-shaped materials deviates from the specified position as the sheet-shaped material runs through the processing device, e.g. a printing machine, a digital printer or copier.
An alignment may be necessary if the existing alignment of the sheet-shaped materials will be changed, such as when a sheet-shaped material that is aligned with an edge will be aligned with a center line or if the orientation will be changed from portrait format to landscape format or vice versa. In the prior art, there are a number of different methods and devices used to achieve an alignment of this type. One group of these devices uses two frictional wheels having the same axis and operated independently of each other with stepper motors. If these two frictional wheels are brought into contact with the sheet-shaped material, the sheet-shaped material experiences a rotary movement if the frictional wheels are operated with angular velocities that are different from each other.
In EP 814 040 B1, in a first phase, a sheet is first turned by an angle in one direction by two frictional wheels then in a second phase it is transported forward, whereby the two frictional wheels move uniformly, and then is turned in reverse in a third phase. During the rotary movements, the angular velocity of one frictional wheel is essentially increased by the amount by which the angular velocity of the other frictional wheel is reduced. After a brief angular velocity change, the rotation process is carried out with two constant angular velocities, after which the angular velocities of the two frictional wheels are adapted again to the transport speed in forward direction. Skewing of a sheet can be compensated, in that the duration of rotation in one direction is selected so that it is different from the duration of rotation in the other direction. A lateral shift is achieved here, since the pivot point of rotation of the first phase is different from the pivot point of rotation of the second phase. A disadvantage here is that a relatively large amount of space is required to achieve the lateral shift by deriving it from a forward movement.
In a related patent EP 0 814 041 B1, a similar method is used in order to cause a change in the orientation of the sheets from a portrait orientation to a landscape orientation, i.e. to rotate the sheet essentially by 90° and at the same time compensate any lateral misalignment or skewed position of the sheets that may exist. Lateral misalignment and skewed position are compensated here by the selection of the suitable pivot point and duration of rotation.